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I watched an episode of grease monkeys where the guy was trying to measure the gauge with a tape measure to figure out what size hole to cut... spent way to much time with no results until someone walked up and showed him the cutouts in the cardboard box.
As long as you aren't using angle rings then it would be a 2 5/8" hole, if using an angle ring then the hole will need to be oblonged in the direction of the angle on both sides.